基于vivado中AXI的模型分析

基于vivado中AXI的模型分析

1、底层代码

`timescale 1 ns / 1 ps

    module myip_v1_0_S00_AXI #
    (
        // Users to add parameters here

        // User parameters ends
        // Do not modify the parameters beyond this line

        // Width of S_AXI data bus
        parameter integer C_S_AXI_DATA_WIDTH    = 32,
        // Width of S_AXI address bus
        parameter integer C_S_AXI_ADDR_WIDTH    = 4
    )
    (
        // Users to add ports here

        // User ports ends
        // Do not modify the ports beyond this line

        // Global Clock Signal
        input wire  S_AXI_ACLK,
        // Global Reset Signal. This Signal is Active LOW
        input wire  S_AXI_ARESETN,
        // Write address (issued by master, acceped by Slave)
        input wire [C_S_AXI_ADDR_WIDTH-1 : 0] S_AXI_AWADDR,
        // Write channel Protection type. This signal indicates the
            // privilege and security level of the transaction, and whether
            // the transaction is a data access or an instruction access.
        input wire [2 : 0] S_AXI_AWPROT,
        // Write address valid. This signal indicates that the master signaling
            // valid write address and control information.
        input wire  S_AXI_AWVALID,
        // Write address ready. This signal indicates that the slave is ready
            // to accept an address and associated control signals.
        output wire  S_AXI_AWREADY,
        // Write data (issued by master, acceped by Slave) 
        input wire [C_S_AXI_DATA_WIDTH-1 : 0] S_AXI_WDATA,
        // Write strobes. This signal indicates which byte lanes hold
            // valid data. There is one write strobe bit for each eight
            // bits of the write data bus.    
        input wire [(C_S_AXI_DATA_WIDTH/8)-1 : 0] S_AXI_WSTRB,
        // Write valid. This signal indicates that valid write
            // data and strobes are available.
        input wire  S_AXI_WVALID,
        // Write ready. This signal indicates that the slave
            // can accept the write data.
        output wire  S_AXI_WREADY,
        // Write response. This signal indicates the status
            // of the write transaction.
        output wire [1 : 0] S_AXI_BRESP,
        // Write response valid. This signal indicates that the channel
            // is signaling a valid write response.
        output wire  S_AXI_BVALID,
        // Response ready. This signal indicates that the master
            // can accept a write response.
        input wire  S_AXI_BREADY,
        // Read address (issued by master, acceped by Slave)
        input wire [C_S_AXI_ADDR_WIDTH-1 : 0] S_AXI_ARADDR,
        // Protection type. This signal indicates the privilege
            // and security level of the transaction, and whether the
            // transaction is a data access or an instruction access.
        input wire [2 : 0] S_AXI_ARPROT,
        // Read address valid. This signal indicates that the channel
            // is signaling valid read address and control information.
        input wire  S_AXI_ARVALID,
        // Read address ready. This signal indicates that the slave is
            // ready to accept an address and associated control signals.
        output wire  S_AXI_ARREADY,
        // Read data (issued by slave)
        output wire [C_S_AXI_DATA_WIDTH-1 : 0] S_AXI_RDATA,
        // Read response. This signal indicates the status of the
            // read transfer.
        output wire [1 : 0] S_AXI_RRESP,
        // Read valid. This signal indicates that the channel is
            // signaling the required read data.
        output wire  S_AXI_RVALID,
        // Read ready. This signal indicates that the master can
            // accept the read data and response information.
        input wire  S_AXI_RREADY
    );

    // AXI4LITE signals
    reg [C_S_AXI_ADDR_WIDTH-1 : 0]     axi_awaddr;
    reg      axi_awready;
    reg      axi_wready;
    reg [1 : 0]     axi_bresp;
    reg      axi_bvalid;
    reg [C_S_AXI_ADDR_WIDTH-1 : 0]     axi_araddr;
    reg      axi_arready;
    reg [C_S_AXI_DATA_WIDTH-1 : 0]     axi_rdata;
    reg [1 : 0]     axi_rresp;
    reg      axi_rvalid;

    // Example-specific design signals
    // local parameter for addressing 32 bit / 64 bit C_S_AXI_DATA_WIDTH
    // ADDR_LSB is used for addressing 32/64 bit registers/memories
    // ADDR_LSB = 2 for 32 bits (n downto 2)
    // ADDR_LSB = 3 for 64 bits (n downto 3)
    localparam integer ADDR_LSB = (C_S_AXI_DATA_WIDTH/32) + 1;
    localparam integer OPT_MEM_ADDR_BITS = 1;
    //----------------------------------------------
    //-- Signals for user logic register space example
    //------------------------------------------------
    //-- Number of Slave Registers 4
    reg [C_S_AXI_DATA_WIDTH-1:0]    slv_reg0;
    reg [C_S_AXI_DATA_WIDTH-1:0]    slv_reg1;
    reg [C_S_AXI_DATA_WIDTH-1:0]    slv_reg2;
    reg [C_S_AXI_DATA_WIDTH-1:0]    slv_reg3;
    wire     slv_reg_rden;
    wire     slv_reg_wren;
    reg [C_S_AXI_DATA_WIDTH-1:0]     reg_data_out;
    integer     byte_index;
    reg     aw_en;

    // I/O Connections assignments

    assign S_AXI_AWREADY    = axi_awready;
    assign S_AXI_WREADY    = axi_wready;
    assign S_AXI_BRESP    = axi_bresp;
    assign S_AXI_BVALID    = axi_bvalid;
    assign S_AXI_ARREADY    = axi_arready;
    assign S_AXI_RDATA    = axi_rdata;
    assign S_AXI_RRESP    = axi_rresp;
    assign S_AXI_RVALID    = axi_rvalid;
    // Implement axi_awready generation
    // axi_awready is asserted for one S_AXI_ACLK clock cycle when both
    // S_AXI_AWVALID and S_AXI_WVALID are asserted. axi_awready is
    // de-asserted when reset is low.

    always @( posedge S_AXI_ACLK )
    begin
      if ( S_AXI_ARESETN == 1'b0 )
        begin
          axi_awready <= 1'b0;
          aw_en <= 1'b1;
        end 
      else
        begin    
          if (~axi_awready && S_AXI_AWVALID && S_AXI_WVALID && aw_en)
            begin
              // slave is ready to accept write address when 
              // there is a valid write address and write data
              // on the write address and data bus. This design 
              // expects no outstanding transactions. 
              axi_awready <= 1'b1;
              aw_en <= 1'b0;
            end
            else if (S_AXI_BREADY && axi_bvalid)
                begin
                  aw_en <= 1'b1;
                  axi_awready <= 1'b0;
                end
          else           
            begin
              axi_awready <= 1'b0;
            end
        end 
    end       

    // Implement axi_awaddr latching
    // This process is used to latch the address when both 
    // S_AXI_AWVALID and S_AXI_WVALID are valid. 

    always @( posedge S_AXI_ACLK )
    begin
      if ( S_AXI_ARESETN == 1'b0 )
        begin
          axi_awaddr <= 0;
        end 
      else
        begin    
          if (~axi_awready && S_AXI_AWVALID && S_AXI_WVALID && aw_en)
            begin
              // Write Address latching 
              axi_awaddr <= S_AXI_AWADDR;
            end
        end 
    end       

    // Implement axi_wready generation
    // axi_wready is asserted for one S_AXI_ACLK clock cycle when both
    // S_AXI_AWVALID and S_AXI_WVALID are asserted. axi_wready is 
    // de-asserted when reset is low. 

    always @( posedge S_AXI_ACLK )
    begin
      if ( S_AXI_ARESETN == 1'b0 )
        begin
          axi_wready <= 1'b0;
        end 
      else
        begin    
          if (~axi_wready && S_AXI_WVALID && S_AXI_AWVALID && aw_en )
            begin
              // slave is ready to accept write data when 
              // there is a valid write address and write data
              // on the write address and data bus. This design 
              // expects no outstanding transactions. 
              axi_wready <= 1'b1;
            end
          else
            begin
              axi_wready <= 1'b0;
            end
        end 
    end       

    // Implement memory mapped register select and write logic generation
    // The write data is accepted and written to memory mapped registers when
    // axi_awready, S_AXI_WVALID, axi_wready and S_AXI_WVALID are asserted. Write strobes are used to
    // select byte enables of slave registers while writing.
    // These registers are cleared when reset (active low) is applied.
    // Slave register write enable is asserted when valid address and data are available
    // and the slave is ready to accept the write address and write data.
    assign slv_reg_wren = axi_wready && S_AXI_WVALID && axi_awready && S_AXI_AWVALID;

    always @( posedge S_AXI_ACLK )
    begin
      if ( S_AXI_ARESETN == 1'b0 )
        begin
          slv_reg0 <= 0;
          slv_reg1 <= 0;
          slv_reg2 <= 0;
          slv_reg3 <= 0;
        end 
      else begin
        if (slv_reg_wren)
          begin
            case ( axi_awaddr[ADDR_LSB+OPT_MEM_ADDR_BITS:ADDR_LSB] )
              2'h0:
                for ( byte_index = 0; byte_index <= (C_S_AXI_DATA_WIDTH/8)-1; byte_index = byte_index+1 )
                  if ( S_AXI_WSTRB[byte_index] == 1 ) begin
                    // Respective byte enables are asserted as per write strobes 
                    // Slave register 0
                    slv_reg0[(byte_index*8) +: 8] <= S_AXI_WDATA[(byte_index*8) +: 8];
                  end  
              2'h1:
                for ( byte_index = 0; byte_index <= (C_S_AXI_DATA_WIDTH/8)-1; byte_index = byte_index+1 )
                  if ( S_AXI_WSTRB[byte_index] == 1 ) begin
                    // Respective byte enables are asserted as per write strobes 
                    // Slave register 1
                    slv_reg1[(byte_index*8) +: 8] <= S_AXI_WDATA[(byte_index*8) +: 8];
                  end  
              2'h2:
                for ( byte_index = 0; byte_index <= (C_S_AXI_DATA_WIDTH/8)-1; byte_index = byte_index+1 )
                  if ( S_AXI_WSTRB[byte_index] == 1 ) begin
                    // Respective byte enables are asserted as per write strobes 
                    // Slave register 2
                    slv_reg2[(byte_index*8) +: 8] <= S_AXI_WDATA[(byte_index*8) +: 8];
                  end  
              2'h3:
                for ( byte_index = 0; byte_index <= (C_S_AXI_DATA_WIDTH/8)-1; byte_index = byte_index+1 )
                  if ( S_AXI_WSTRB[byte_index] == 1 ) begin
                    // Respective byte enables are asserted as per write strobes 
                    // Slave register 3
                    slv_reg3[(byte_index*8) +: 8] <= S_AXI_WDATA[(byte_index*8) +: 8];
                  end  
              default : begin
                          slv_reg0 <= slv_reg0;
                          slv_reg1 <= slv_reg1;
                          slv_reg2 <= slv_reg2;
                          slv_reg3 <= slv_reg3;
                        end
            endcase
          end
      end
    end    

    // Implement write response logic generation
    // The write response and response valid signals are asserted by the slave 
    // when axi_wready, S_AXI_WVALID, axi_wready and S_AXI_WVALID are asserted.  
    // This marks the acceptance of address and indicates the status of 
    // write transaction.

    always @( posedge S_AXI_ACLK )
    begin
      if ( S_AXI_ARESETN == 1'b0 )
        begin
          axi_bvalid  <= 0;
          axi_bresp   <= 2'b0;
        end 
      else
        begin    
          if (axi_awready && S_AXI_AWVALID && ~axi_bvalid && axi_wready && S_AXI_WVALID)
            begin
              // indicates a valid write response is available
              axi_bvalid <= 1'b1;
              axi_bresp  <= 2'b0; // 'OKAY' response 
            end                   // work error responses in future
          else
            begin
              if (S_AXI_BREADY && axi_bvalid) 
                //check if bready is asserted while bvalid is high) 
                //(there is a possibility that bready is always asserted high)   
                begin
                  axi_bvalid <= 1'b0; 
                end  
            end
        end
    end   

    // Implement axi_arready generation
    // axi_arready is asserted for one S_AXI_ACLK clock cycle when
    // S_AXI_ARVALID is asserted. axi_awready is 
    // de-asserted when reset (active low) is asserted. 
    // The read address is also latched when S_AXI_ARVALID is 
    // asserted. axi_araddr is reset to zero on reset assertion.

    always @( posedge S_AXI_ACLK )
    begin
      if ( S_AXI_ARESETN == 1'b0 )
        begin
          axi_arready <= 1'b0;
          axi_araddr  <= 32'b0;
        end 
      else
        begin    
          if (~axi_arready && S_AXI_ARVALID)
            begin
              // indicates that the slave has acceped the valid read address
              axi_arready <= 1'b1;
              // Read address latching
              axi_araddr  <= S_AXI_ARADDR;
            end
          else
            begin
              axi_arready <= 1'b0;
            end
        end 
    end       

    // Implement axi_arvalid generation
    // axi_rvalid is asserted for one S_AXI_ACLK clock cycle when both 
    // S_AXI_ARVALID and axi_arready are asserted. The slave registers 
    // data are available on the axi_rdata bus at this instance. The 
    // assertion of axi_rvalid marks the validity of read data on the 
    // bus and axi_rresp indicates the status of read transaction.axi_rvalid 
    // is deasserted on reset (active low). axi_rresp and axi_rdata are 
    // cleared to zero on reset (active low).  
    always @( posedge S_AXI_ACLK )
    begin
      if ( S_AXI_ARESETN == 1'b0 )
        begin
          axi_rvalid <= 0;
          axi_rresp  <= 0;
        end 
      else
        begin    
          if (axi_arready && S_AXI_ARVALID && ~axi_rvalid)
            begin
              // Valid read data is available at the read data bus
              axi_rvalid <= 1'b1;
              axi_rresp  <= 2'b0; // 'OKAY' response
            end   
          else if (axi_rvalid && S_AXI_RREADY)
            begin
              // Read data is accepted by the master
              axi_rvalid <= 1'b0;
            end                
        end
    end    

    // Implement memory mapped register select and read logic generation
    // Slave register read enable is asserted when valid address is available
    // and the slave is ready to accept the read address.
    assign slv_reg_rden = axi_arready & S_AXI_ARVALID & ~axi_rvalid;
    always @(*)
    begin
          // Address decoding for reading registers
          case ( axi_araddr[ADDR_LSB+OPT_MEM_ADDR_BITS:ADDR_LSB] )
            2'h0   : reg_data_out <= slv_reg0;
            2'h1   : reg_data_out <= slv_reg1;
            2'h2   : reg_data_out <= slv_reg2;
            2'h3   : reg_data_out <= slv_reg3;
            default : reg_data_out <= 0;
          endcase
    end

    // Output register or memory read data
    always @( posedge S_AXI_ACLK )
    begin
      if ( S_AXI_ARESETN == 1'b0 )
        begin
          axi_rdata  <= 0;
        end 
      else
        begin    
          // When there is a valid read address (S_AXI_ARVALID) with 
          // acceptance of read address by the slave (axi_arready), 
          // output the read dada 
          if (slv_reg_rden)
            begin
              axi_rdata <= reg_data_out;     // register read data
            end   
        end
    end    

    // Add user logic here

    // User logic ends

    endmodule

上面的代码源自vivado自动生成的代码，仅用于学习。

2、逐个端口信号分析

（1）基本命名规律

端口全部大写，无论是传递参数还是端口输入输出，全部大写。内部信号小写，用于区分是否需要将数据传递出去。

AW：adress write的简写，写地址通道的所属信号。

W：write的简写，写数据通道所属信号。

B：写响应通道的所属信号，为什么用B尚不清楚，简单记忆为back，返回信号。

AR：adress read的简写，读数据通道所属信号

R：read 的简写，读数据通道所属信号。

VALID：握手信号的一部分

READY：握手信号的一部分

（2）每个通道的信号分析

AW：ADDR-地址写入的载体信号。PORT-仲裁信号，确定主机写入是否可行。VALID-主机准备信号。READY-从机准备信号。

W：DATA-数据写入的载体信号。STRB-数据写入位数信号。

B：RESP-写响应输出信号（也是写信号是否完成的信号）。

AR：ADDR-读数据载体信号。PORT-仲裁信号

R：DATA-数据读取的载体信号。RESP-读通道状态。

注：只是列举了一个通道的握手信号，其他通道的作用是一致的。

注意：areset是A RESET的简写，不是AR信号，不要误解。

3、逐个always块分析

 （1）aw模块分析

基本所有always模块第一个if都是复位判断，后面不再详细说明。从这个模块可以分析出从机的aw开始工作是主机的ar和r都准备完毕。从机aw的结束是写响应通道握手成功时，aw_en置1，写地址通道释放。写地址的内容通过awaddr暂存。

（2）w模块分析

从机的w是在主机的r和ar都准备好后且从机的b开始工作后的停止。

（3）数据写入缓存模块分析

这里总共有4个寄存器，通过aw通道的addr和w通道的strb两个信号构成的简易选择器选择输入。

（4）b模块分析

这里的载体信号resp没有实际工作，一直为0，没有进行判断。这和串口通信总是不加奇偶校验位是一个德行。从机的握手信号有效是前面的步骤完成了之后，握手成功后即关闭。

（5）ar模块分析

直接判断握手是否成功，成功即将地址存到addr。握手成功后回到等待状态。

（6）r模块分析

同样是需要从机的ar通道握手成功后开始读取数据。r通道依靠resp表明是否可以读取，这里也省略了这里的设计。数据通过从机的r通道的data发送。

（7）数据读取缓存模块

读取的数据通过ar通道addr来选择某个缓存器。原理和写数据缓存差不多。也是一个选择器的功能。

注：我这里为了方便理解，将从机的r对应主机的w，从机的w对应从机的r（aw和ar是一样的），实际设计中可能不是这样，毕竟没有接触过主机的设计逻辑。

4、物理模型对应

上面紫色的和棕色的序号分别代表数据的写和读的全部流程，也是对应上述设计的主要信号的。这是AXI协议最简单的传输方式，也是最简单的架构。lite的AXI4实现功能的方式就是这么直接。

5、总结

这次通过简单分析AXI4的lite构型，为下面自定义AXI4的FPGA提供基本的理论以及设计规范的支持。